1. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $13.79 to $14.59, inclusive. The Reporting Person undertakes to provide to the Issuer, any security holder of the Issuer or the staff of the United States Securities Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the ranges set forth in this footnote (1) to this Form 4.

2. The reportable securities are owned directly by Polaris Venture Partners V, L.P. ("PVP V"). Polaris Venture Management Co. V, L.L.C. ("PVM V") is the general partner of PVP V. The Reporting Person, a member of the Issuer's Board of Directors, is a member of PVM V. Each of Jonathan A. Flint ("Flint") and Terrance G. McGuire ("McGuire") are the managing members of PVM V. Each of Flint, McGuire and the Reporting Person, in their respective capacities with respect to PVM V, may be deemed to have shared voting and dispositive power over the shares held by PVP V. Each of PVM V, Flint, McGuire and the Reporting Person disclaims beneficial ownership of these securities and this report shall not be deemed an admission that any one of them is the beneficial owner of such securities for purposes of Section 16 or for any other purpose, except to the extent of their respective pecuniary interests therein.

3. The reportable securities are owned directly by Polaris Venture Partners Entrepreneurs' Fund V, L.P. ("PVPE V"). PVM V is the general partner of PVPE V. The Reporting Person, a member of the Issuer's Board of Directors, is a member of PVM V. Each of Flint and McGuire are the managing members of PVM V. Each of Flint, McGuire and the Reporting Person, in their respective capacities with respect to PVM V, may be deemed to have shared voting and dispositive power over the shares held by PVPE V. Each of PVM V, Flint, McGuire and the Reporting Person disclaims beneficial ownership of these securities and this report shall not be deemed an admission that any one of them is the beneficial owner of such securities for purposes of Section 16 or for any other purpose, except to the extent of their respective pecuniary interests therein.

4. The reportable securities are owned directly by Polaris Venture Partners Founders' Fund V, L.P. ("PVPFF V"). PVM V is the general partner of PVPFF V. The Reporting Person, a member of the Issuer's Board of Directors, is a member of PVM V. Each of Flint and McGuire are the managing members of PVM V. Each of Flint, McGuire and the Reporting Person, in their respective capacities with respect to PVM V, may be deemed to have shared voting and dispositive power over the shares held by PVPFF V. Each of PVM V, Flint, McGuire and the Reporting Person disclaims beneficial ownership of these securities and this report shall not be deemed an admission that any one of them is the beneficial owner of such securities for purposes of Section 16 or for any other purpose, except to the extent of their respective pecuniary interests therein.

5. The reportable securities are owned directly by Polaris Venture Partners Special Founders' Fund V, L.P. ("PVPSFF V"). PVM V is the general partner of PVPSFF V. The Reporting Person, a member of the Issuer's Board of Directors, is a member of PVM V. Each of Flint and McGuire are the managing members of PVM V. Each of Flint, McGuire and the Reporting Person, in their respective capacities with respect to PVM V, may be deemed to have shared voting and dispositive power over the shares held by PVPSFF V. Each of PVM V, Flint, McGuire and the Reporting Person disclaims beneficial ownership of these securities and this report shall not be deemed an admission that any one of them is the beneficial owner of such securities for purposes of Section 16 or for any other purpose, except to the extent of their respective pecuniary interests therein.

Remarks:

/s/ Max Eisenberg Attorney-in-Fact for Amir Nashat

02/01/2019

** Signature of Reporting Person

Date

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.

* If the form is filed by more than one reporting person,
see
Instruction
4
(b)(v).